There are a couple of technical reasons, but I don't know if there is a regulatory source.  Vibration is one concern and I was told that steel should be used on each end of a hose that connects between the engine to a stationary location.  Steel has an infinite fatigue life and aluminum has a finite fatigue life that depends on stress level.  Is this a sufficient reason to use steel?  I doubt that the vibratory stresses in most fittings could ever be high enough to be a concern - after all, the hose itself isolates the movement.  The second reason is fire protection - in theory the aluminum fittings could melt in the case of an engine fire.  I doubt that this would happen in the case of a fairly massive fitting, but I suppose it could happen to a -3 or -4 fitting used for a gage line exposede to direct flame (no internal flow to cool the fitting).  I'm sure I added a pound or two of weight by using steel fittings, but that's what I did.